Lignin products are widely employed as additives in various chemical processes and compositions. Lignin is a complex, high-molecular weight polymer occurring naturally in close association with cellulose in plants and trees. In the paper-making industry, lignin may be recovered as a by-product of the cellulose product by two principal wood-pulping processes known as the sulfite process and the kraft process. In the sulfite process, lignin is solubilized from the cellulosic portion of the wood pulp by direct sulfonation, while the kraft process is based on an alkaline degradation mechanism causing cleavage of .beta.-aryl ether linkages in the polymeric lignin which sequentially result in chemical functions of the phenolic and carboxylic type. Kraft process lignin generally is isolated by acid precipitation from the black liquor of the pulping process at a pH below the pKa of the phenolic groups.
Depending on conditions under which a kraft lignin is precipitated, the lignin may be either in the form of a free acid lignin or a lignin salt. If the lignin is-precipitated at a high pH, such as about 9.5 to 10, the lignin is obtained in the form of a salt. If this lignin is further processed by washing, acidifying to a low pH, and further washed to be substantially free of salt and ash-forming ingredients, free acid lignin, known as "A" lignin, is obtained.
The high degree of chemical activity characteristic of lignin permits the preparation of many organic derivatives. Lignin by-products variously are employed in other chemical compositions as a surfactant, extender, dispersant, reinforcement, absorbent, binder, sequestering agent, emulsifier, emulsion stabilizer, and stabilizing and protective colloid. Lignosulfonate compounds, particularly sodium salts of lignosulfonates, are widely employed as additives and dispersants in textile dyestuffs and printing pigments, and sodium salt sulfonated lignin by-products have been sold for many years under the trademarks Polyfon.RTM. and REAX.RTM. by Westvaco Corporation of North Charleston, S.C.
Dilling U.S. Pat. No. 4,590,262 owned by Westvaco Corporation discloses an improved method of producing sodium salts of low electrolyte-containing lignosulfonates suited for use as dye and print paste additives comprising the steps of ionizing the phenol component of the lignin material in an alkaline liquid medium, methylolating the ionized phenol component of the lignin by addition of an aldehyde, such as formaldehyde, lowering the pH of the liquid medium to an acid pH to precipitate the methylolated lignin material, washing the precipitated lignin material with water to remove inorganic salts and residual reactants therefrom, and subsequently sulfonating the washed purified methylolated lignin material with a sodium salt of a sulfur-oxygen-containing compound, such as sodium bisulfite.
Under processing conditions described in U.S. Pat. No. 4,590,262, using the molar amounts of formaldehyde and the sodium-oxygen-containing compound (sodium bisulfite) described, sulfonation of the lignin occurs at the methylolation site on the aromatic phenolic ring of the lignin molecule, which is referred to as sulfomethylation of the lignin. As stated, the sulfomethylation reaction preferably is conducted at atmospheric pressure and at a temperature at around 80-100.degree. C., although elevated pressures and temperatures above 100.degree., up to about 190.degree. C., may be employed.
U.S. Pat. No. 4,590,262 also discloses that it is possible to sulfonate the lignin side chain of the aromatic nucleus by sodium sulfite treatment in the absence of formaldehyde. Low sulfonated lignin products of the invention of U.S. Pat. No. 4,590,262, i.e., sodium salts of sulfomethylated lignins having a degree of organically bound sulfonation of about 1 mole or less per 1,000 grams of lignin may be produced, and higher sulfomethylated lignin products, i.e., having a sulfomethylation of greater than about 1.6 moles per 1,000 grams of lignin, may also be produced.